1. Field of the Invention
The present invention relates to a light extraction substrate for an organic light-emitting device (OLED), and more particularly, to a method of fabricating a light extraction substrate for an OLED by which the flatness of the light extraction substrate for an OLED can be increased.
2. Description of Related Art
In general, an organic light-emitting device (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into a hole injection layer and then migrate from the hole injection layer through a hole transport layer to the organic light-emitting layer, and electrons are injected from the cathode into an electron injection layer and then migrate from the electron injection layer through an electron transport layer to the light-emitting layer. Holes and electrons that are injected into the light-emitting layer recombine with each other in the light-emitting layer, thereby generating excitons. When such excitons transit from the excited state to the ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on the mechanism that drives the N*M number of pixels which are arranged in the shape of a matrix.
In an active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
However, as shown in FIG. 8, only about 20% of light generated by an OLED is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate 10 and an organic light-emitting layer 30 which includes an anode 20, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer and an electron injection layer and by a total internal reflection originating from the difference in the refractive index between the glass substrate 10 and the air. Specifically, the refractive index of the internal organic light-emitting layer 30 ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode 20 ranges from 1.9 to 2.0. Since the two layers have a very small thickness ranging from 100 to 400 nm and the refractive index of glass used for the glass substrate 10 is about 1.5, a planar waveguide is thereby formed inside the OLED. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate 10 is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate 10, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate 10. Since the ratio of the trapped light is up to about 35%, only about 20% of the generated light is emitted to the outside. Herein, reference numerals 31, 32 and 33 indicate components of the organic light-emitting layer 30. Specifically, 31 indicates the hole injection layer and the hole transport layer, 32 indicates the light-emitting layer, and 33 indicates the electron injection layer and the electron transport layer.
The typical method for overcoming the problem of the light trapping was to increase external light extraction efficiency using a microlens array. However, the microlens array is easily damaged by external impacts or be contaminated by impurities since convex-concave portions of a light extraction layer protrude outward. When the microlens array is used in a display, it also has the problem of blurred images caused by lenses.
In addition, a light extraction layer which changes an optical waveguide path is provided between the glass substrate 10 and the anode 20 in order to increase internal light extraction. Such internal light extraction layer extracts light that would otherwise be lost by the waveguiding effect, and thus the possibility of increasing the internal light extraction efficiency is significantly higher than the possibility of increasing the external light extraction efficiency. In order to increase the light extraction efficiency of the internal light extraction layer, the surface of the light extraction layer must have a convex-concave structure. In this case, however, the anode 20 which adjoins to the light extraction layer must have a similar shape. This increases a danger in that the anode 20 has a localized sharp portion. When the anode 20 has a sharp protruding portion, the current is concentrated in the sharp portion. This consequently causes a leakage current or lowers the power efficiency.
In order to overcome this problem, several approaches were proposed in the related art, among which there was a method of impregnating light-scattering particles in a matrix material. This method includes properly mixing a matrix material and light-scattering particles and then coating the glass substrate 10 with a mixture of the matrix material and the light-scattering particles, for example, by spin coating or bar coating or using a slot die. When the matrix material is implemented as a metal oxide, a sol is also used. However, the use of the sol disadvantageously causes evaporation and/or loss in an organic matter included in the sol during drying and curing, thereby reducing the volume to a range from 1/10 to 1/20 of the thickness of a metal oxide thin film that is initially formed. In addition, since the light-scattering particles do not shrink during the drying or curing, the light-scattering particles protrude from the surface of the metal oxide thin film that is shrunk. This disadvantageously increases the surface roughness of the metal oxide thin film that is supposed to act as an internal light extraction layer that requires high flatness. The increased surface roughness of the metal oxide thin film has an adverse effect on the longevity of an OLED that uses the metal oxide thin film as an internal light extraction layer.
The information disclosed in the Background of the Invention section is provided only for or better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.